THE DELIVERY OF MARINE-DERIVED NUTRIENTS TO LOTIC ECOSYSTEMS: AN INTER-ANNUAL AND BETWEEN-SYSTEM STUDY.

A two-year study was conducted in Southeast Alaska to examine the influence of marine-derived nutrients (MDN) on food web dynamics. Freshwater fishes, invertebrates, and biofilm were collected above and below a salmon migration barrier, pre- and post-salmon spawning, in two streams with different hydrological regimes. Our study utilized the distinct isotopic values of returning salmon to delineate the biological transfer of MDN. After spawning, carbon and nitrogen values for invertebrates and fish were higher below the barrier (e.g. Heptageniidae, -22.3 and 8.3 per mil respectively) than above the barrier (-30.6 and 0.2 per mil) reflecting MDN incorporation. Pre-spawning values were also elevated below the barrier (-26.1 and 1.6 per mil) relative to above (-30.4 and 0.2 per mil), suggesting a “legacy” effect from annual MDN deposition. Interestingly, invertebrate trophic relationships appeared similar between years. The inter-annual dataset enabled us to examine isotopic values and relate changes to potential differences in salmon escapement levels. Investigating inter-annual and between-system isotopic variation is a necessary step to determine if ecological responses show similar or divergent patterns.